Add basic frontend assets with case-studies page

- Add IE specific stylesheets
- Add them to the pre-compile list in `config/initalizers/assets.rb`
- Add them to the application layout
Set-up slimmer correctly

- Use header_footer_only rather than everything
- Use asset path for the assets to work in production
- Use Plek to locate static for slimmer
- Enable caching on slimmer in production
- Add basic layout Sass so page is still centered.
More work
Create presenter for content_item

This will need to be split at some point in the not to distant future
into a case study presenter and a generic edition like presenter. But
for the moment it is in a generic content item presenter.

Also use components to make the page closer represent the direction it
is heading in.